/* Primzahlenzwillinge finden basierend mit dem Sieb des Erathostenes,
   basierend auf der Sieb-Implementierung von Al Aburto */
#include <stdio.h>
#include <stdlib.h>

char *sieve(long m)
{
  register char *flags;
  register long i,prime,k;
  register long size;
  
  long  j;
  
  char *ptr;
  

  size  = m - 1;
  ptr   = malloc(m);

   if( !ptr ) {
     perror("malloc failed");
     exit(1);
   }

   flags = ptr;
   
   j = 0;

   for(i=0 ; i<=size ; i++) {                     
     *(flags+i) = 1;                              
   }                                              
                                                  
   for(i=0 ; i<=size ; i++) {                     
     if(*(flags+i)) {                             
       prime = i + i + 3;                         
       for(k = i + prime ; k<=size ; k+=prime) {
         *(flags+k)=0;                        
       }                                        
       
     }
   }                                            
   return flags;
}                                              

long twins(char *flags, long m)
{
  long i, r;
  for (i=0, r=0; i<m-1; i++) {
    if (flags[i] && flags[i+1])
      r++;
  }
  return r;
}

int main(int argc, char *argv[])
{
  long m, limit;
  char *s;
  if (argc != 2) {
    fprintf(stderr, "Usage: %s <n>\n", argv[0]);
    exit(1);
  }
  limit = atol(argv[1]);
  m = (limit-1)/2;
  s = sieve(m);
  printf("%ld Primzahl-Zwillinge <= %ld\n",twins(s,m),limit);
  return 0;
}